Centrifugal fan and noise reduction method in centrifugal fan

ABSTRACT

A centrifugal fan having a sound reduction effect for noises in a wide frequency range is provided. The centrifugal fan comprises a motor to which an impeller is connected, a casing which surrounds the impeller and which has a bell-mouth-like suction port, and a bell-mouth-like orifice which has a suction port being concentric with the suction port of the casing and having a diameter as large as that of the suction port of the casing, in an outer shell, providing a plurality of protrusions in a resonant space surrounded by the orifice, the casing, and the outer shell, and providing depressions which are formed so as to be interposed between the protrusions.

TECHNICAL FIELD

The present invention relates to a centrifugal fan for use in ventilation fan equipment or the like and a centrifugal fan noise reduction method of reducing noises caused by operation of the centrifugal fan.

BACKGROUND ART

Conventionally, a centrifugal fan of this type has been known that is used in a centrifugal fan or the like and that has an orifice, separate from a suction casing having a bell-mouth-like suction port, on an opening on one face of an outer shell thereof (see Patent Literature 1, for instance).

Hereinbelow, the conventional centrifugal fan 100 will be described with reference to FIG. 10.

As shown in FIG. 10, the centrifugal fan 100 has an outer shell 101 that has one face opened, an impeller 103 that is supported rotatably by a top surface 102 in the outer shell 101, a motor 104 that is fixed to the top surface 102 and that is for driving and rotating the impeller 103, a scroll casing 105 that surrounds the impeller 103, and a suction casing 107 that has a suction port 106. The centrifugal fan 100 further includes an orifice 109 that has a suction hole 108 having a same size as or smaller size than the suction port 106, at a specified distance h from the suction casing 107. The orifice 109 is placed with a gap i provided between a lower end 110 of the suction casing 107 and an end 111 of the orifice 109. Thus a resonant space 112 communicating with the suction port 106 through an inlet part 115 between the lower end 110 of the suction casing 107 and the end 111 of the orifice 109 is defined by the outer shell 101, the scroll casing 105 and the orifice 109. The centrifugal fan 100 further includes a grill 113 placed on one face of the orifice 109 and a discharge port 114 provided on one side surface of the outer shell 101.

In such a configuration of the conventional centrifugal fan 100, the rotation of the impeller 103 causes sucked air to pass through the grill 113 and the suction hole 108 of the orifice 109, to enter the impeller 103 via the suction port 106 of the suction casing 107, to undergo a pressure increase through agency of the impeller 103, to pass through inside of the scroll casing 105, and to be discharged through the discharge port 114. In this process, acoustic waves of rotation noises caused with the pressure increase in the impeller 103, turbulent flow noises caused by vortices that are produced by the passage through the scroll casing 105, noises produced in the scroll casing 105, and the like are radiated from the suction port 106, and a portion of the acoustic waves is incident on the resonant space 112 from the inlet 115 having the gap i. In the acoustic waves of the incident noises, acoustic waves of the noises having frequencies specified by a capacity, a shape and the like of the resonant space 112 undergo air column resonance in the resonant space 112, reduction in acoustic wave level, and resonance silencing.

Patent Literature 1: JP 3279834 B

DISCLOSURE OF INVENTION Problems to be Solved by the Invention

Such a conventional centrifugal fan is capable of reducing noises having comparatively low frequencies not higher than about 1 kHz by the resonance silencing effect, whereas such a fan is poor in effect of reducing noises having comparatively high frequencies not lower than about 1 kHz, such as noises caused by turbulent flow. Accordingly, there is an issue in that a quantity of reduction of noises is decreased on condition that noises having high frequencies are dominant therein, e.g., on condition that a large quantity of blow is attained by the centrifugal fan. In centrifugal fans, therefore, it is demanded to increase an effect of reducing noises in a high frequency band and to reduce noises in a wide frequency range from low frequency to high frequency.

The invention resolves such conventional issues, and an object of the invention is increase the effect of reducing noises in the high frequency band in a centrifugal fan and to provide a centrifugal fan that is capable of reducing noises in a wide frequency range from low frequency to high frequency.

Means to Solving the Issue

In order to achieve the object, the invention is configured as follows.

According to a first aspect of the present invention, there is provided a centrifugal fan comprising:

an outer shell having an opening portion,

an impeller that is supported rotatably in the outer shell,

a motor for driving and rotating the impeller,

a casing that has a first suction port and that is placed in the outer shell so as to surround the impeller, and

a bell-mouth-like orifice that is placed on the opening portion of the outer shell so as to have a gap between the orifice and the first suction port of the casing and that has a second suction port providing communication between the opening portion of the outer shell and the first suction port of the casing, wherein

a resonant space that communicates with the first suction port through the gap between the first suction port and the second suction port and that effects resonance silencing against noises emitted from the first suction port is defined by the orifice, the casing and the outer shell that serve as definition walls, and wherein a plurality of protruding portions are formed on the definition walls for the resonant space.

According to a second aspect of the present invention, there is provided a centrifugal fan as defined in the first aspect, wherein

the plurality of protruding portions comprise sloped surfaces sloping relative to a surface of the definition walls for the resonant space in positions where the protruding portions are formed, and wherein

the sloped surface of one protruding portion and the sloped surface of another protruding portion adjacent to the one protruding portion are connected to each other so that a recessed part is formed by the sloped surfaces between both the protruding portions.

According to a third aspect of the present invention, there is provided a centrifugal fan as defined in the second aspect, wherein connected parts of the sloped surfaces in each recessed part form an acute angle.

According to a fourth aspect of the present invention, there is provided a centrifugal fan as defined in the second aspect, wherein the protruding portions each have a flat surface on a top part thereof.

According to a fifth aspect of the present invention, there is provided a centrifugal fan as defined in the second aspect, wherein the protruding portions each have a shape such that a depth thereof is longer than a height and a width thereof.

According to a sixth aspect of the present invention, there is provided a centrifugal fan as defined in the fifth aspect, wherein a plurality of protruding portion groups in each of which a plurality of protruding portions are arranged with depthwise directions thereof being parallel to one another are arranged so that depthwise directions of adjacent protruding portion groups intersect with each other.

According to a seventh aspect of the present invention, there is provided a centrifugal fan as defined in claim 5, wherein a plurality of protruding portion groups in each of which a plurality of protruding portions are arranged with depthwise directions thereof being parallel to one another are arranged so as to have different depthwise directions.

According to an eighth aspect of the present invention, there is provided a centrifugal fan as defined in the fifth aspect, wherein the plurality of recessed parts formed between the plurality of protruding portions are formed so as to extend along the surfaces of the definition walls and are arranged so that directions in which the recessed parts extend are different from at least one of radial directions, a circumferential direction and an axial direction of the impeller.

According to a ninth aspect of the present invention, there is provided a centrifugal fan as defined in any one of the first through eighth aspects, wherein the plurality of protruding portions have the same shape and wherein the plurality of recessed parts formed between the plurality of protruding portions have the same shape.

According to a tenth aspect of the present invention, there is provided a centrifugal fan as defined in any one of the first through eighth aspects, wherein the protruding portions have a plurality of different shapes and wherein the recessed parts formed between the plurality of protruding portions have a plurality of different shapes.

According to an eleventh aspect of the present invention, there is provided a centrifugal fan as defined in any one of the first through eighth aspects, wherein the plurality of protruding portions are provided on the orifice in the resonant space.

According to a twelfth aspect of the present invention, there is provided a centrifugal fan as defined in any one of the first through eighth aspects, wherein the plurality of protruding portions are formed integrally with the definition walls of the orifice, the casing and the outer shell.

According to a 13th aspect of the present invention, there is provided a centrifugal fan as defined in any one of the first through eighth aspects, wherein the protruding portions are detachably provided on the definition walls.

According to a 14th aspect of the present invention, there is provided a centrifugal fan as defined in the 13th, wherein the protruding portions are formed of sound absorbing material.

According to a 15th aspect of the present invention, there is provided a centrifugal fan as defined in any one of the first through eighth aspects, wherein the protruding portions are formed so as to have cavities inside and wherein holes that provide communication between the cavities and the resonant space are formed on the protruding portions.

According to a 16th aspect of the present invention, there is provided a noise reduction method in a centrifugal fan comprising an outer shell having an opening portion, an impeller that is supported rotatably in the outer shell, a casing that has a first suction port and that is placed in the outer shell so as to surround the impeller, and a bell-mouth-like orifice that is placed on the opening portion of the outer shell so as to have a gap between the orifice and the first suction port of the casing and that has a second suction port providing communication between the opening portion of the outer shell and the first suction port of the casing, the method comprising:

making acoustic waves of noises occurring in the casing with rotational drive of the impeller incident, through the gap between the first suction port and the second suction port, on a resonant space that communicates with the first suction port through the gap between the first suction port and the second suction port and that is defined by the orifice, the casing and the outer shell that serve as definition walls, and

reducing acoustic wave levels of the noises incident on the resonant space by air column resonance, and reducing acoustic wave levels of the noises by diffused reflection of the acoustic waves of the noises in the resonant space.

EFFECT OF THE INVENTION

The centrifugal fan of the invention employs the configuration including the resonant space communicating with the first suction port of the casing and the plurality of protruding portions in the resonant space. With the employment of such a configuration, acoustic wave levels of acoustic waves of noises incident on the resonant space can be reduced by the air column resonance and can be reduced by occurrence of the diffused reflection caused by the plurality of protruding portions. Therefore, the two functions of the air column resonance and the diffused reflection increase the sound reduction effect in the high frequency band for noises caused by operation of the centrifugal fan and ensure reduction of noises in a wide frequency range from low frequency to high frequency.

According to the noise reduction method for the centrifugal fan in accordance with the invention, noises caused by operation of the centrifugal fan are made incident on the resonant space communicating with the first suction port of the casing of the centrifugal fan, and acoustic wave levels of the noises incident on the resonant space can be reduced by the air column resonance. In addition, the acoustic wave levels can be reduced by the diffused reflection of the acoustic waves of the noises in the resonant space. Therefore, the two functions of the air column resonance and the diffused reflection increase the sound reduction effect in the high frequency band for the noises caused by the operation of the centrifugal fan and ensure reduction of noises in a wide frequency range from low frequency to high frequency.

BRIEF DESCRIPTION OF DRAWINGS

These aspects and features of the present invention will become clear from the following description taken in conjunction with the preferred embodiments thereof with reference to the accompanying drawings, in which:

FIG. 1A is a schematic section of a centrifugal fan in accordance with a first embodiment of the invention;

FIG. 1B is an enlarged view of a part X of the centrifugal fan of FIG. 1A;

FIG. 2A is a schematic perspective view of an orifice in accordance with the first embodiment of the invention;

FIG. 2B is a section of the orifice taken along line A-A in FIG. 2A;

FIG. 3 is a schematic perspective view showing shapes of protrusions and depressions in the centrifugal fan in accordance with the first embodiment of the invention;

FIG. 4 is a diagram showing a noise reduction effect in the centrifugal fan in accordance with the first embodiment of the invention;

FIG. 5 is a schematic perspective view of an orifice in accordance with a modification of the first embodiment of the invention;

FIG. 6 is a schematic section of a centrifugal fan in accordance with the modification of the first embodiment of the invention;

FIG. 7A is a schematic perspective view of an orifice in accordance with a second embodiment of the invention;

FIG. 7B is a section of the orifice taken along line B-B in FIG. 7A;

FIG. 8 is a schematic perspective view of an orifice in accordance with a third embodiment of the invention;

FIG. 9A is a schematic section showing protrusions and depressions in a centrifugal fan in accordance with a fourth embodiment of the invention;

FIG. 9B is a schematic diagram showing an example in which cavities are configured in the protrusions in the centrifugal fan in accordance with the fourth embodiment of the invention; and

FIG. 10 is a schematic section showing a conventional centrifugal fan.

DESCRIPTION OF EMBODIMENTS

Before the description of the present invention proceeds, it is to be noted that like parts are designated by like reference numerals throughout the accompanying drawings.

Hereinbelow, embodiments of the invention will be described in detail with reference to the drawings.

First Embodiment

FIG. 1A shows a schematic section of a centrifugal fan 1 in accordance with a first embodiment of the invention, and FIG. 1B shows an enlarged fragmentary view of a part X in FIG. 1A. As shown in FIG. 1A, the centrifugal fan 1, which is used as a ceiling embedded type ventilating fan, for instance, has a motor 6 to which a multiblade impeller 5 is connected so as to be capable of rotating about a rotation shaft 4, and a casing 10 surrounding the impeller 5, having a bell-mouth-like suction port 7 (first suction port) on an opening thereof and having a discharge port 9 on a side wall 8 thereof, in an outer shell 3 having an opening 2 on a bottom face thereof. The discharge port 9 of the casing 10 communicates with a discharge adapter 12 through a discharge opening 11 provided on one side surface of the outer shell 3. A flange 13 exists on an outer circumference of the bottom face of the outer shell 3, which is fixed to ceiling material 15 by screws or the like through holes 14 provided in the flange 13, and a duct 16 provided on the ceiling and communicating with outdoors is joined to the discharge port 9 through the discharge adapter 12.

On the opening 2 of the outer shell 3 is provided an orifice 18 having a bell-mouth-like suction port 17 (second suction port) concentric with the suction port 7 of the casing 10. That is, the opening 2 of the outer shell 3 communicates with the suction port 7 of the casing 10 through the suction port 17 of the orifice 18. As shown in FIGS. 1A and 1B, an end 20 of the orifice 18 is positioned with a gap j from a lower face of the suction port 7 of the casing 10, and a gap part (clearance) 21 is formed between the orifice 18 and the casing 10.

On an outer circumference of the orifice 18 is formed a resonant space 19 surrounded and defined by the orifice 18, the casing 10 and the outer shell 3, which serve as definition walls. The resonant space 19 communicates with the suction port 7 of the casing 10 through the gap part 21 between the orifice 18 and the casing 10.

A plurality of protrusions (protruding portions) 22 are formed on the definition walls in the resonant space 19, and depressions (recessed parts) 23 are formed between adjacent protrusions 22. FIG. 2A shows a schematic perspective view of the orifice 18, and FIG. 2B shows a section of the orifice 18 taken along line A-A in FIG. 2A. As shown in FIGS. 2A and 2B, the plurality of protrusions and depressions 23 are continuously formed on the orifice 18 on a side facing inside of the resonant space 19. In the first embodiment, the protrusions 22 and the orifice 18 are provided as one component formed integrally. The protrusions 22 each have a shape such that a size along its depthwise direction (one direction in a plane) is longer than a size along its widthwise direction in plan view. The protrusions 22 having such a shape longer in the depthwise direction are provided, on the orifice 18, as a plurality of protrusion groups (protruding portion groups) in each of which a set of three protrusions 22 are arranged with the depthwise directions thereof being parallel to one another, and the protrusions 22 are arranged so that the depthwise directions of adjacent protrusion groups 30 are generally orthogonal to each other as shown in FIG. 2A.

FIG. 3 shows an enlarged view of the protrusions 22. As shown in FIG. 3, the protrusions 22 each have a shape such that a protrusion depth L (length in a direction Y shown in the drawing) is longer than a protrusion height ho (height in a direction Z shown in the drawing). The protrusions 22 each have a trapezoidal shape in a section (X-Z section) perpendicular to a direction (Y-direction) of the depth thereof. A top part of each protrusion 22 is formed of a flat surface 31, and sloped surfaces 32 sloping relative to the Z-direction are formed on both side surfaces (both side surfaces facing in the X-directions) of each protrusion 22 so that the top flat surface 31 having a width a spreads by a width c to both sides with respect to a downward direction. Each depression 23 having a section generally in shape of a letter V defined by the two sloped surface 32 is formed between two protrusions 22 arranged with the depthwise directions thereof being parallel to each other. A width of a top opening of the depression 23 is designated by a character b and the width of the depression 23 gradually decreases with respect to the downward Z-direction so that the two sloped surface 32 intersect with each other at a depression angle α. The depression angle α is an acute angle.

Hereinbelow will be described a function of reducing noises caused by the rotational drive of the impeller 5 through agency of the resonant space 19 and the plurality of protrusions 22 provided in the resonant space 19 in the centrifugal fan 1 having such a configuration. In the centrifugal fan 1, the rotation of the impeller 5 effected by the motor 6 causes sucked air to pass through the main body opening 2 and the suction port 17 of the orifice 18, to enter the impeller 5 via the suction port 7 of the casing 10, to undergo a pressure increase through agency of the impeller 5, to pass through inside of the casing 10, and to be discharged through the discharge adapter 12 into the duct 16 and then to outdoors.

In this process, acoustic waves of rotation noises caused with the pressure increase in the impeller 5, turbulent flow noises caused by vortices that are produced by the passage through the casing 10, noises amplified by resonance in the casing 10, and the like are radiated downward from the suction port 7. A portion of the acoustic waves of the noises radiated from the suction port of the casing 10 is incident on the resonant space 19 through the gap part 21 formed between the end 20 of the orifice 18 and the casing 10. A portion of the incident acoustic waves is made into of antiphase acoustic waves and is returned to the gap part 21 by fixed end reflection on wall surfaces (definition wall surfaces) in the resonant space 19. Thus there occurs such resonance silencing resulting from air column resonance as cancels out a portion of the acoustic waves radiated from the suction port 7, so that acoustic wave levels of the noises can be reduced. A frequency of the acoustic waves that are cancelled out is determined by a distance of a path of the acoustic waves from the gap part 21 to a position where the reflection occurs. The existence of a large number of protrusions 22 in the resonant space 19 facilitates reflection on the large number of protrusions 22 of the acoustic waves incident on the resonant space 19, and a variety of distances between the gap part 21 and the protrusions 22 make it possible to reduce noises having various frequencies.

A portion of the acoustic waves having collided with the protrusions 22 may travel toward adjacent protrusions 22 after the reflection, depending on angles of incidence thereof, and may gradually be reduced in energy in the depressions 23 by diffused reflection with subsequent repetition of reflection, so that the noises can be reduced.

Adjacent protrusions 22 are continuously arranged so that each depression 23 has the section generally in shape of the letter V, and the depressions 23 are provided so as to have a small width. Thus the acoustic waves incident on the depressions 23 are reflected a larger number of times, and the resultant diffused reflection strengthens the sound reduction effect, thereby reducing the noises. The protrusion height ho and the width b of the depression openings preferably have a relation ho b, for instance.

Each protrusion 22 has the trapezoidal shape in the section perpendicular to the direction of the depth of the protrusion 22, so that the depressions 23 formed by the protrusions 22 gradually narrow with respect to the downward direction. Therefore, acoustic waves incident on the depressions 23 undergo diffused reflection. The lower the acoustic waves travel in the depressions 23, the shorter time intervals of occurrence of the reflection become. This results in an increase in the number of times of the reflection, strengthened action of the sound reduction effect, and reduction of the noises. In the trapezoidal section of each protrusion 22, the protrusion height ho and the width c of the sloped surfaces 32 preferably have a relation ho≧c, for instance.

With use of protrusions 22 having triangular sections, similar effects can be expected. With use of the protrusions 22 having the trapezoidal sections, however, capacities of the depressions 23 can be adjusted by adjustment in a width a of the top flat surfaces 31 of the protrusions 22 while the angle of the sloped surfaces 32 is kept at an optimum angle. Achievement of the resonance silencing based on air column resonance requires optimum setting of a capacity of the resonant space 19, and the optimization of the capacity requires the adjustment in the capacities of the depressions 23. Accordingly, the protrusions 22 having the trapezoidal sections facilitate the adjustment for the optimization of the capacity of the resonant space 19. The sloped surfaces of the protrusions forming the depressions 23 are not limited to flat surfaces, and similar effects can be expected with use of the sloped surfaces formed of curved surfaces.

The depression angle α of the depressions 23 that is set as an acute angle facilitates diffused reflection of acoustic waves having shallow angles of incidence on the depressions 23 and obtainment of the sound reduction effect, resulting in effective reduction of noises.

The protrusions 22 have the shape such that the depth thereof is longer than the height and width thereof, and thus the depressions 23 formed by the protrusions 22 are shaped like slits. Thus diffused reflection can be effected even for acoustic waves having shallow (small) angles of incidence with respect to the direction (i.e., the Y-direction (see FIG. 3)) of the depths and slits of the depressions 23. This facilitates obtainment of the sound reduction effect and results in achievement of noise reduction. The protrusion depth L, the protrusion height ho, and the protrusion extremity width a preferably have a relation L>1.5ho or L>1.5a, for instance.

On condition that the shapes of the protrusions 22 are identical and that the shapes of the depressions 23 formed by the protrusions 22 are identical, the sound reduction effect strongly acts on a specified frequency and the relevant noises can be reduced because frequencies of the acoustic waves that undergo sound reduction caused by diffused reflection on the depressions 23 depend on the shapes of the depressions 23.

The acoustic waves that come into the resonant space 19 through the gap part 21 tend to be diffracted toward the orifice 18 in the resonant space 19 by the end 20 of the orifice 18 (see FIGS. 1A and 1B). This increases a possibility that the acoustic waves collide with the orifice 18. With the formation of the plurality of protrusions 22 and depressions 23 on the orifice 18, therefore, the acoustic waves can effectively be made incident on the depressions 23 and the noises can be reduced by the sound reduction effect of the depressions 23.

As shown in FIG. 2A, the protrusions 22 and the orifice 18 are formed as one component formed integrally, so that the plurality of protrusions 22 and depressions 23 can be provided in the resonant space 19 without increase in number of components. The manufacture can be facilitated and a cost for the manufacture can be reduced, by provision of such a sectional shape of the orifice 18 as can be manufactured by a unidirectional blanking die, as shown in FIG. 2B, for instance.

With the employment of the configuration in which the plurality of protrusion groups 30 each composed of the set of three protrusions 22 are arranged alternately lengthwise and breadthwise, at maximum four slit-like depressions 23 arranged in parallel can be formed in one protrusion group 30. There is a characteristic in that an incidence of the diffused reflection on the depressions 23 is the higher for acoustic waves in a direction the closer to a direction perpendicular to a longitudinal direction of the slit-like depressions 23, and provision of, e.g., four or more protrusions 22 as one set strengthens the characteristic. Though the incidence of the diffused reflection is comparatively low for acoustic waves nearly parallel to the longitudinal direction of the slit-like depressions 23, the alternately lengthwise and breadthwise arrangement on the orifice 18 of the protrusion groups 30 each forming the set of four protrusions 22 can compensate for the low incidence with regard to the adjoining sets. Therefore, the diffused reflection on the depressions 23 can effectively be brought about so that the sound reduction effect can be obtained.

FIG. 4 shows a graph of noise characteristic values of the centrifugal fan 1 of a working example in accordance with the first embodiment, the fan having internal sizes of the outer shell 3 of 264 mm square, a height thereof of 195 mm, a diameter of the impeller 5 of 183.5 mm, an inside diameter Di of the suction port 7 of 152 mm, an inside diameter Do of the suction port 17 of 152 mm, the gap j of 5 mm, the protrusion height ho of 20 mm, the protrusion top flat surface width a of 14 mm, the protrusion depth L of 66 mm, and the depression angle α of 41°, for instance, and of a centrifugal fan of a comparative example provided with no protrusion 22 (other conditions are the same as those of the working example). A solid line therein represents noise characteristic values of the centrifugal fan of the working example having the protrusions 22 and the depressions 23, and a dotted line therein represents noise characteristic values of the centrifugal fan of the comparative example having neither protrusions nor depressions. In the noise characteristic of the working example, as is apparent from FIG. 4, it is found that sound pressure levels at various frequencies are reduced and that, in particular, sound reduction in comparatively high frequency bands of 1 kHz to 1.4 kHz and of 2.6 kHz to 3.4 kHz is remarkable in comparison with the comparative example. Thus it can be observed that the employment of the configuration of the centrifugal fan in accordance with the first embodiment provides an overall noise reduction effect of 1 dB.

As shown in FIG. 5, the protrusion groups 30 can be positioned so that the depthwise directions of the protrusions 22 of the protrusion groups 30 are arranged along directions inclined relative to directions parallel to radial directions of the impeller 5 and relative to a direction orthogonal to the radial directions. In such a configuration, effects similar to those of the above configuration can be expected. Any angles can be employed as inclination angles of the inclined directions.

Though the configuration of the centrifugal fan 1 of FIG. 1 in which the plurality of protrusions 22 are formed on the orifice 18 in the resonant space 19 has been described, a configuration in which the protrusions 22 are formed on the outer shell 3 and the casing 10 as well as the orifice 18 as shown in FIG. 6 can be employed in place of the described configuration. The employment of such a configuration facilitates the sound reduction resulting from air column resonance and diffused reflection and thus furthers the sound reduction effect, so that noises can effectively be reduced.

The first embodiment provides the centrifugal fan that is capable of increasing the effect of reducing sound in a high frequency band and reducing noises in a wide frequency range from low frequency to high frequency, by use in combination of the sound reduction effect resulting from air column resonance and the sound reduction effect resulting from diffused reflection.

Second Embodiment

Hereinbelow will be described a centrifugal fan in accordance with a second embodiment of the invention. In the following description, the same component members as those of the first embodiment are designated by the same reference numerals, and description thereof is omitted. FIG. 7A shows a schematic perspective view of an orifice 18 that the centrifugal fan in accordance with the second embodiment includes, and FIG. 7B shows a section of the orifice 18 taken along line B-B in FIG. 7A.

In FIG. 7A, a plurality of protrusions 22 are discontinuously formed on the orifice 18 on a side facing inside of a resonant space 19, that is, so that a side surface of a protrusion 22 is not directly connected to a side surface of an adjacent protrusion 22, and depressions 23 formed of the side surfaces and bottom surfaces of the adjacent protrusions 22 are provided between the protrusions 22. The protrusions 22 and the orifice 18 are formed as one component formed integrally. The protrusions 22 differ in shape and, namely, the protrusions 22 having a plurality of shapes are provided. Accordingly, the depressions 23 formed by the protrusions 22 also have a plurality of shapes. As shown in the section of FIG. 7B, for instance, the plurality of protrusions 22 are spaced apart from one another and are discontinuously formed without direct connections among the side surfaces thereof. Besides, the depressions 23 are arranged at desired intervals with respect to the radial directions of the impeller 5. The depressions 23 are placed so that longitudinal directions of slit-like shapes thereof extend in various directions.

On condition that acoustic waves of noises that have come in through the gap part 21 travel in various directions through agency of diffraction, reflection and/or the like inside the resonant space 19, employment of the arrangement configuration of the protrusions 22 and the depressions 23 of the second embodiment ensures a sound reduction effect and noise reduction for the acoustic waves in the various directions because the depressions 23 are placed so that the longitudinal directions of the slit-like shapes thereof extend in various directions.

The configuration in which the protrusions 22 have the plurality of shapes and in which the depressions formed by the protrusions 22 have the plurality of shapes is employed because frequencies of the acoustic waves that undergo sound reduction caused by diffused reflection on the depressions 23 depend on the shapes of the depressions 23. Such a configuration ensures the sound reduction effect for various frequencies and noise reduction in a wider frequency range in comparison with the configuration of the first embodiment. As long as the longitudinal directions of the protrusions 22 and the depressions 23 are arranged so as to differ from at least any one of the radial directions, a circumferential direction and an axial direction of the impeller 5, the sound reduction effect can be obtained for the acoustic waves in various directions.

The second embodiment provides the centrifugal fan that is capable of increasing the effect of reducing sound in a high frequency band and reducing noises in a wide frequency range from low frequency to high frequency, by use in combination of the sound reduction effect resulting from air column resonance and the sound reduction effect resulting from diffused reflection.

Third Embodiment

Hereinbelow will be described a centrifugal fan in accordance with a third embodiment of the invention. In the following description, the same component members as those of the first embodiment are designated by the same reference numerals, and description thereof is omitted. FIG. 8 shows a schematic perspective view of an orifice 18 that the centrifugal fan in accordance with the third embodiment includes.

As shown in FIG. 8, a plurality of protrusion groups 30 in which protrusions 22 having the same shape are provided as each protrusion group 30 are arranged alternately lengthwise and breadthwise on the orifice 18 on a side facing inside of a resonant space 19, in a manner similar to the arrangement of the protrusions 22 of the first embodiment shown in FIG. 2A. The first embodiment employs the configuration in which the plurality of protrusion groups 30 are formed integrally with the orifice 18, whereas the third embodiment employs a configuration different from the first embodiment in that each protrusion group is configured as a detachable unit 24. Specifically, each unit 24 (corresponding to the protrusion group 30) includes three protrusions 22 of which depthwise directions are arranged in parallel and two slit-like depressions 23 which are formed by the protrusions 22 and which are parallel to the depthwise directions. On the orifice 18 on the side facing inside of the resonant space 19, a plurality of units 24 are provided arbitrarily lengthwise and breadthwise so that the depthwise directions of adjacent units 24 intersect with each other. Each unit 24 is fixed to the orifice 18 by fixation means such as screws and is thus detachable from the orifice 18.

The employment of such a configuration makes it possible to provide arrangement configuration of the units 24 in various manners and to produce various arrangements of the depressions 23 with use of the units 24 having one type of shape. With use of the detachable units 24, frequencies for which sound reduction can be attained can be adjusted by alteration in the arrangement of the depressions 23.

Use of the units 24 made of sound absorbing material such as glass wool furthers gradual decrease in energy caused by diffused reflection on the depressions 23 and thus results in increased action of the sound reduction effect and attainment of noise reduction.

The third embodiment provides the centrifugal fan that is capable of increasing the effect of reducing sound in a high frequency band and reducing noises in a wide frequency range from low frequency to high frequency, by use in combination of the sound reduction effect resulting from air column resonance and the sound reduction effect resulting from diffused reflection, and that allows the adjustment in frequencies of noises that can be reduced.

Fourth Embodiment

Hereinbelow will be described a centrifugal fan in accordance with a fourth embodiment of the invention. In the following description, the same component members as those of the first embodiment are designated by the same reference numerals, and description thereof is omitted. FIGS. 9A and 9B show schematic sections illustrating structures of protrusions 22 and depressions 23 provided in the centrifugal fan in accordance with the fourth embodiment.

As shown in FIG. 9A, the protrusions 22 of the fourth embodiment have cavities 25 therein with a plate thickness, e.g., of 1 mm and have holes 27, e.g., of φ1.5 mm on protrusion extremity faces 26. The protrusions 22 form a Helmholtz resonance structure that includes the cavities 25 as capacity parts and the holes 27 as throat parts. In such a structure of the protrusions 22, the cavities 25 can easily be formed inside the protrusions 22 by formation of the top faces of the protrusions 22 using the orifice 18 formed of a thin plate and by attachment of a plate part 28 from downside of the orifice 18 in the drawing, e.g., as shown in FIG. 9B.

In such a configuration of the fourth embodiment, the cavities 25 are formed inside and the holes 27 are formed therein, in addition to the depressions 23 that effect sound reduction. Thus addition of resonance silencing effected by the Helmholtz resonance structure ensures a further sound reduction effect and noise reduction.

Therefore, the fourth embodiment provides the centrifugal fan that is capable of increasing the effect of reducing sound in a high frequency band and reducing noises in a wide frequency range from low frequency to high frequency.

It is to be noted that, by properly combining the arbitrary embodiments of the aforementioned various embodiments, the effects possessed by them can be produced.

The invention provides the centrifugal fans that are capable of increasing the sound reduction effect in a high frequency band and reducing noises in a wide frequency range from low frequency to high frequency, and thus can be applied to air conditioning equipment, ventilation fan equipment and the like.

Although the present invention has been fully described in connection with the preferred embodiments thereof with reference to the accompanying drawings, it is to be noted that various changes and modifications are apparent to those skilled in the art. Such changes and modifications are to be understood as included within the scope of the present invention as defined by the appended claims unless they depart therefrom.

The disclosure of specifications, drawings, and claims of Japan Patent Application No. 2008-111032 filed on Apr. 22, 2008 is incorporated herein by reference in its entirety. 

1. A centrifugal fan comprising: an outer shell having an opening portion, an impeller that is supported rotatably in the outer shell, a motor for driving and rotating the impeller, a casing that has a first suction port and that is placed in the outer shell so as to surround the impeller, and a bell-mouth-like orifice that is placed on the opening portion of the outer shell so as to have a gap between the orifice and the first suction port of the casing and that has a second suction port providing communication between the opening portion of the outer shell and the first suction port of the casing, wherein a resonant space that communicates with the first suction port through the gap between the first suction port and the second suction port and that effects resonance silencing against noises emitted from the first suction port is defined by the orifice, the casing and the outer shell that serve as definition walls, and wherein a plurality of protruding portions are formed on the definition walls for the resonant space.
 2. The centrifugal fan as defined in claim 1, wherein the plurality of protruding portions comprise sloped surfaces sloping relative to a surface of the definition walls for the resonant space in positions where the protruding portions are formed, and wherein the sloped surface of one protruding portion and the sloped surface of another protruding portion adjacent to the one protruding portion are connected to each other so that a recessed part is formed by the sloped surfaces between both the protruding portions.
 3. The centrifugal fan as defined in claim 2, wherein connected parts of the sloped surfaces in each recessed part form an acute angle.
 4. The centrifugal fan as defined in claim 2, wherein the protruding portions each have a flat surface on a top part thereof.
 5. The centrifugal fan as defined in claim 2, wherein the protruding portions each have a shape such that a depth thereof is longer than a height and a width thereof.
 6. The centrifugal fan as defined in claim 5, wherein a plurality of protruding portion groups in each of which a plurality of protruding portions are arranged with depthwise directions thereof being parallel to one another are arranged so that depthwise directions of adjacent protruding portion groups intersect with each other.
 7. The centrifugal fan as defined in claim 5, wherein a plurality of protruding portion groups in each of which a plurality of protruding portions are arranged with depthwise directions thereof being parallel to one another are arranged so as to have different depthwise directions.
 8. The centrifugal fan as defined in claim 5, wherein the plurality of recessed parts formed between the plurality of protruding portions are formed so as to extend along the surfaces of the definition walls and are arranged so that directions in which the recessed parts extend are different from at least one of radial directions, a circumferential direction and an axial direction of the impeller.
 9. The centrifugal fan as defined in claim 1, wherein the plurality of protruding portions have the same shape and wherein the plurality of recessed parts formed between the plurality of protruding portions have the same shape.
 10. The centrifugal fan as defined in claim 1, wherein the protruding portions have a plurality of different shapes and wherein the recessed parts formed between the plurality of protruding portions have a plurality of different shapes.
 11. The centrifugal fan as defined in claim 1, wherein the plurality of protruding portions are provided on the orifice in the resonant space.
 12. The centrifugal fan as defined in claim 1, wherein the plurality of protruding portions are formed integrally with the definition walls of the orifice, the casing and the outer shell.
 13. The centrifugal fan as defined in claim 1, wherein the protruding portions are detachably provided on the definition walls.
 14. The centrifugal fan as defined in claim 13, wherein the protruding portions are formed of sound absorbing material.
 15. The centrifugal fan as defined in claim 1, wherein the protruding portions are formed so as to have cavities inside and wherein holes that provide communication between the cavities and the resonant space are formed on the protruding portions.
 16. A noise reduction method in a centrifugal fan comprising an outer shell having an opening portion, an impeller that is supported rotatably in the outer shell, a casing that has a first suction port and that is placed in the outer shell so as to surround the impeller, and a bell-mouth-like orifice that is placed on the opening portion of the outer shell so as to have a gap between the orifice and the first suction port of the casing and that has a second suction port providing communication between the opening portion of the outer shell and the first suction port of the casing, the method comprising: making acoustic waves of noises occurring in the casing with rotational drive of the impeller incident, through the gap between the first suction port and the second suction port, on a resonant space that communicates with the first suction port through the gap between the first suction port and the second suction port and that is defined by the orifice, the casing and the outer shell that serve as definition walls, and reducing acoustic wave levels of the noises incident on the resonant space by air column resonance, and reducing acoustic wave levels of the noises by diffused reflection of the acoustic waves of the noises in the resonant space. 